Gain control for seismograph amplifiers



Aug- 28, 1.945 E. M. sHooK 2,383,571

GAIN CONTROL FOR SEISMOGRAPH AMPLIFIERS Original Filed Feb. 1, 1938 4Sheets-Sheet S Aug.v 2s, 1945. E M. SHOCK 2,383,571

' GAIN CONTROL FOR SEISMQGRAPH AMPLIFIERS Original Filed Feb. l, 1938 4Sheets-Sheet 4 www@ www

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Patented ug. 28, 1945 OFFICE.

GAIN CONTROL FOR SEISMOGRAPH AIVIPLIFIERS Earley M. shook, washington-D.c., assigner, by 'mesne assignments, to Socony-Vacuum Oil Company,Incorporated, corporation of New York `New York, N. Y., a4

continuation or' applicati@ serial No. 188,067,

February 1,

1943, serial No. 4s3,z12

1`4 claims. (c l. 177-352) This invention relates generally to a .methodand apparatus for surveying the subsurface strata of the earth by theuse of articially created seismic waves, and more particularly to themethod and apparatus for recording all the data on a single spread whenshot in one direction by detonating a single charge of explosive, and isa continuation of my application Serial No. 188,067, illed February 1,1938, for Gain control for seismograph amplifiers.

In the art of exploring subsurface strata by the use of articiallycreated seismic waves, it is customary to detonate a charge ofexplosives at a point on or near the earths surface and record theseismic waves generated by the detonation of.

the explosives at predetermined points removed from the point ofgeneration. From the data thus recorded it is possible to ascertain thedepth of subsurface horizons, from which the seismic waves that arerecorded have bee'n reflected. The depths of these subsurface horizonsare computed from the velocity and time required for these seismic wavesto travel down to the reflecting horizon and return to the detectinglinstrument. This velocity will vary, dependent upon the density andelastic coeillcients of the materials through which it travels. i Inaddition to considering the velocity at which reflected waves willtravel in subsurface strata, it is necessary to consider the Velocity oftheir transmission through the unconsolidated,-

` weathered, sedimentary surface of the earth.

The usual procedure when the exploration of an ,area is begun is todevelop these velocities in the different strata by what is termed avelocity Program which entails recording records of seismic waves whichwill give directly the velocities in these particular strata. Afterhaving once ascertained these velocities, they can be used throughoutthe area in computing the depth of the partlcula`r reflecting strata.Due-to the fact that the weathered surface layer of the earth varies inthickness, it is necessary that the thickness of this rweatheredlayer becomputed for each spread or location of geophones. To obtain the datafrom winch' the thickness of the weathered layer is computed, charges ofexplosives are detonated at the selected s'hot points and the velocityof the wave travel through the weathered or surfacelayer is determined.Due to the fact that the wave in which we are llrst interested travelssubstantially in a direct route to the detector, there is very littleabsorption present and an impulse e of great amplitude is recorded. Itis necessary that the gain in amplication of a vacuum tube amplifier beat the maximum in order that the 1938. This application April 15,

break in the seismogram trace, resulting from the arrival at thegeophone of the first impulse explosives are detonated Waves.

-as lmuch as 20 or 30 of the of energy, be very definite. As aconsequence heretofore, it has been necessary to record these data on aseparate seismogram from that on which reflected waves are recorded.Such a procedute would necessitate recording a plurality of seismogramsto obtain the data in the weathered layer and records of the reflectedwaves from shallow subsurface strata. Since the'high cost of fieldoperations makes the time factorrof paramount importance. it isdesirable to record all of these data on a single record of standardwidth from thev detonation of a single charge of explosives. Such aprocedure would result in tremendous savings in the amount of explosivesrequired and in the time required by the field party to record the datanecessary on a single spread, as Well as the time required for aninterpreter to observe the data .on a plurality of seismograms.

With previous methods of recording these data by the use of a pluralityof charges of explosives, from which seismic waves have been recorded ona corresponding number -of seismograms, it has been almost `impossibleto duplicate the conditions under which each charge of explosive isdetonated, due to several factors, the most,

important of which being the changes in the conditions affecting themedium in which the shot is planted When one charge of explosives isdetonated in the bottom of a shot hole, a pocket or cavity is formed insome formations, while in others the bottom of the hole is lllled up,due to the fact that the medium forming the walls of the cavity is ofloosely packed material. 'I'his sometimes results in a loss of as muchas` 10.'I in the depth of the hole, and, in extremecases, where thebottom of the hole is in a quicksand material, hole are lost lby thedetonation of a single charge in it. Such changes in conditions underwhich successive charges of introduce variable factors inthe apparentvelocities of the recorded files commited fromv these data in error.

Therefore, the primary object of this invention is the provision of amethod and apparatus whereby all the data that is required to berecorded on a particular spread when shot from one Thesevariable-factors would make pro- Still another object of this inventionis in the provision of means whereby the vacuum tube amplifier isallowed to remain at or'near its maximum sensitivity until the firstimpulse of the direct traveling waves has been recorded; .theninstantaneously suppressing the gain in amplification and allowing it toexpand eir-A ponentially with time.

This invention further contemplates means ofexplosives.

Still another object ofl this invention resides in the provision of amethod for controlling the gain in amplification in a predeterminedmanner, without sacrificing any of the available gain present in theamplifier at the time the first impulses are recorded from directtraveling waves or at the time the latest reflected waves are recorded.I

Still another object of this invention is in the provision of a meansfor initiating the predetermined control which can be accomplishedautomatically by the operation of a gaseous discharge tube,` or byrelay', or manually by switch.

'Ihis invention also contemplates the provision of means which willoperate on either the control or suppressor grid of a conventionalvacuum tube in one or more stages of the conventional amplifier toefiect the above mentioned results.

Another object of the `invention is in the provision of a unit foreffecting these results which can be utilized with a conventionalamplifier.

Still another object of the invention resides in rthe provision of aunit which will operate with low energy consumption.

Other objects and advantages will become apparent from the followingdetailed description, when considered with the attached drawings, in

which:

Figures 1a and 1b.' placed end to end, illustrate a seismogram recordedin the conventional manner;

Figures 2a and 2b similarly illustrate a seismogram of the type that canbe recorded by the method and with the apparatus of this invention;

Figures 3a and 3b, placed end to end, illustrate a voltage wave thatwould result from passing a wave of constant amplitude through anelectric seismograph amplifier employing the control forming the subjectmatter of this invention;

Figure 4 is a curve which has been plotted with gain in amplification asordinates and time las abscissae representing the instantaneous Figure 8is the circuit diagram shown in Figure 7 showing additionally the use ofa gas triode vacuum tube for the purposeof initiating the operation ofthe control;

Figure 9 is a partial block diagram of an electric seismograph showingthe application of the circuit forming the subject matter of thisinvention to a conventional amplifier.

Referring to the drawings in detail, particularly Figures 1a and 1bthere is illustrated a seismogram-which shows the' manner in which thedirect traveling waves, as well as the reflected wavesI from fivegeophones or detectors are conventionally recorded on' a singleseismogram. The transverse lines indicate denite and equal smallintervals of time, such, for example, as .01 second. It will be notedthat there is an indication known in the art as a time break on themiddle trace at X- indicating the instant at which the charge ofexplosive was detonated. The period of time elapsing between the instantat which the charge was detonated and the arrival of the waves generatedthereby at the geophone is represented by the number of spaces andfractions thereof between the indication X, denoting the time break orinstant of explosion, and the point where a discontinuity occurs on therespective traces. To record a record such as that illustrated thecharge of explosive and the five geophones are located in the samevertical plane and the geophones are lequidistantly spaced one from theother. The discontinuity in the respective traces is indicative of theinstant at which the first energy strikes the geophone. Although thisfirst initial impulse is generated by direct traveling waves of abruptwave front, there are many other Waves of different frequencies andvelocities that follow the wave which delivers the initial impulse tothe detector. Due to the fact that these direct traveling waves are ofso much greater amplitude than reflected waves, their attenuated wavetrain would tend to obscure reflected waves from shallow horizons thatwould arrive at the geophone or detector a relatively short interval oftime after' the first impulse. As shown by the illustration in Figures1a and 1b, it is impossible to distinguish any of the reflected wavesfrom shallow horizons, from direct traveling Waves. This is partly dueto the fact that the direct traveling waves completely obscure thereflected waves, and partly due to the fact that there is lessabsorption and resulting loss of energy in waves which have beenreflected from shallow horizons than those which have been reflectedfrom deep horizons. From this 'it readily becomes apparent that somemeans must be provided for controlling both the amplitude of the directtraveling waves after the first impulse has been recorded and theamplitude of reected waves from shallow horizons. Such control ofamplification is illustrated in Figures 2a and 2b. These figuresillustrate a seismogram such as would be recorded by the method andapparatus forming the subject matter of this invention. In thisillustration it will be noted that the first arrival of energy to thedetector or geophone is recorded on the seismogram at an amplitudecorresponding to that disclosed in Figure 1a. Additionally, it will benoted that, after the last first break has been recorded, the gain inamplification in all amplifiers has been suppressed to some small value,which, in some cases, can be zero. Then, from this point the gain inamcorrelate or proiile. In some areas the weathered v layer or overlyingunconsolidated stratum is relay tively' thin and reflectionsarerobtainabie from shallow horizons underlying the weathered layer. Ifit is impossible to instantly suppress the gain in amplification afterthe direct traveling waves have been recorded, these shallow reectionswill be' completely obscured. Therefore, the utility for this particularresponse is readily apparent. A record such as that illustrated inFigures `2a and 2b in addition to giving-edenite data from which thethickness of the weathered layer can be checked at each'spread, alsogives the reflections from all horizons that it is desired towork orprole. Since these are all the data that are required from -seismicwaves which have been recorded in one direction on a particular spread,there is no necessity for detonating more than one charge of explosiveand recording more than one seismogram. This obviously results in atremendous saving of time, explosives, and a saving in depreciation ofthe equipment.

To obtain a seismogram such as that illustrated in Figures 2d and 2b, itis desired that the amplification characteristic of the electric orvacuum tube amplifier be as disclosed in Figures 3a and 3b' and inFigure 4 in which there is shown a curve having the gain in amplicationas ordinates plotted against time as abscissae.

A fundamental electrical circuit which may be used in connection withthe control or suppressor grid of a. conventional vacuum tube in one ormore stages of a vacuum tube ampliiier, is shown in Figure 5. Thiscircuit, although a simple circuit, serves very well vto illustrate thefundamental principles of this invention. In this circuit there isillustrated, connected in series, a source of potential I, a resistanced, a condenser 5 and a switch 2. Then, connectedin parallel with theseries combination of resistance d and capacity 5, there is a. secondresistance 3. At thepoint 6 in the circuit a ground wire is attached,bringing this point in the circuit tozero potential. When the switch 2is closed, a source Cof potential l will cause a current to flow in thecircuit through the resistance 4 until the condenser 5 is fully charged.The intensity of the `current flowing during the charging process of ingtiine for thecondenser 5 is controllable by the. Aresistance 4.- Fromthe fundamental elements of electric circuits it is well known that acurv rent flowing through a resistance is in time phase mum negativevalue instantly, and then from its maximum negative value to zeroexponentially with time. In order to complete a discharge circuitfor'the condenser 5, a resistance 3 is connected as described above andshown in Figure 5. This discharge circuit becomes effective to restorethe circuit to original conditions and makes it ready for a repetitionof the cycle described above when the switch 2 is open.

The varying potential at the point B can be utilized as a variable gridbias voltage on a vacuum tube in an electric seismograph amplier.

It is well known that the gain *n amplification plitlcation of thisvacuum tube. the battery S is connected in series with the pointproduced by a vacuum tube can ne controlled by varying the grid biasvoltage. Additionally, it has been found that if a sumciently largenegative value of voltage is impressed across the grid of a vacuum tube,it can be biased completely out of its characteristic and no appreciablegain can be obtained from that tube. Dependent upon the 'type and makeof a particular vacuum tube used for purposes of amplification in anamplifier, it will function to produce the greatest gain at a zero gridpotential4 or some small negative-value of grid bias voltage.Additionally, it has been found that -by varying this grid bias fromavzero or small negative value to a relatively large negative value, theamplification can be reduced proportionally, and this principle isutilized in con-l trolling the instantaneous values of gain inamplication in the seismograph amplifier.

In Figure 'l there is shown this fundamental circuit connected to thecontrol grid of a conventional vacuum tube of a conventional amplifier.The simple circuit explained above has now been modified in that theresistance has been divided into two variable resistances 4 and l inseries.

The point B is made variable Ion the resistance d in order to select'the desired negative potential and the pointA is made variable in orderto control the charging time for the condenser 5. By connecting thepoint B tothe control -grid of a vacuum tube, the variable potentialdescribed above can be utilized to control the gain in am- Additionally,if

B and the control grid of a vacuum tube 9, one is enabled to control theinitial and/ or maximum `grid bias voltage, which, in `turn functions tocontrol the gain in amplification produced by the tube 9. As describedabove, when the switch 2 in this circuit is closed, the point B isimmediately placed at a negative potential with respect to the ground 6.This negativepotential is se-v lected suliciently large that the grid isbiased out of the characteristic of the tube suillciently far to reducethe gain in amplification to a predetermined value. This is effectedinstantly lon closing the switch 2. As the condenser 5 aprelationshipwith the voltage across the resistance, y

Consequently, the IR drop across the resistance 4,'while the condenser 5is charging, has similar lcharacteristics to that of the current flowingthrough it. Therefore, the potential at the point B with respect to thepointv 6, which is grounded, goes through the same values relative totime that the 1R drop across the resistance ,6 does. That is. thispotential will go from -zero value to its maxiproaches full charge, thecurrent in theresistances 4 and 1 approaches zero, and the potentialfrom the ground, of the point B as a result approaches zero, causing thegrid bias voltage on the tube 9 to assume the initial value. Theposition o f the point A on the resistance I can be so yadjusted as tocontrol the time required to asthe time required to record a record,such as that disclosed in Figures 2a and 2b. ComparingA the curve to theseismogram illustrated in Figures 2a and 2b, it will be noted thatbetween the points G and` H on the curve, the gain in amplification isat a constant predetermined value. 'I'his period of time represents thetime required for the initial breaks due to the direct traveling seismicwaves to be recorded. At the instant at which the last initial breakoccurs on the trace from the geophone most distant from the shot, thegain in amplification immediately drops to a value represented by thepoint K. This drop in. gain of amplification occurs instantly with theclosing of the switch 2 discussed above. Dependent upon the value of theresistances 4 and 1, the capacity of the condenser 5 and the magnitudeof the potential I, the gain in amplification is then caused to expandexponentially with time as vthe condenser 5 is fully charged during aninterval of time corresponding to the distance between the points K andL on the curve.

In Figure 8 there is illustrated a circuit which corresponds to that inFigure '7, with the exception that the operation of the circuit tocontrol the gain in amplification of the vacuum tube 9 is initiated byagas triode tube I3, such as the tube commonly sold on the market underthe trade-name of Thyratron The-characteristics of a. vacuum tube ofthis type are well known to one skilled in the art; that is, that a gridplaced between a cathode and anode of a hot cathode tube containing gas,if sufficiently-negative, will prevent the arc from striking, becauseunder these conditions, there is no accelerated field around thecathode. -The electrons, therefore, cannot acquire sufiicient velocityto ionize. When the grid is made positive, electrons are acceleratedtoward it. They collide withgas atoms, ionizing them and the arcstrikes. Therefore, it can be seen that a negative grid prevents the arcfrom striking, and a positive grid allows it to strike, effecting a flowof current from the cathode to the anode of the tube.

After the arc has struck, it presents the peculiar feature that the gridhas no more control of it, and the current will continue to flow untilthe plate circuit is broken. As shown, the grid is negatively biasedwith respect to its cathode- A by the bias battery C.

The arc in the gaseous discharge tube can be caused to strike in manyways. For example, the impulse of current flowing in the blastercircuitv that is dependedv upon for the time-break signal, can be usedto change the grid bias voltage of the gaseous discharge tube from alarge negative value to a positive or small negative value, or theinitial impulse of the wave train after sufficient amplification, can beused for the same purpose In other words, a potential from any source,just so long as it has a suicien'tly high positive value, can be usedt'o initiate the flow of current in the plate circuit of the gaseousdischarge tube.

After the arc is struck in the tube, the current from the battery I orplate supply source which flows/in the plate circuit causes an IR dropacross resistance 3. This IR drop is impressed across l. scribed abovein completing the charging circuit for the condenser 5, switches l0 andIl are p roy vided. Switch I0 is used for opening the gas triode platecircuit, to stop the flow of current in the plate circuit, for it is acharacteristic of the gas triode tube that once the arc is struck, thegrid has no more control. v Switch I I is used for removing the biasmanually from the grid of the gas trlode tube to cause current to flowin its plate circuit for test purposes. A resistance IIa is included inthe circuit completed by switch I I. Anfter all adjustments have beenmade and a seismogram is to be recorded, the 'normal position forv theswitch Il) is closed and the switch II is open.

When a positive potential resulting from an impulse is impressed on thegrid of the gas triode tube to reduce the negative bias from battery C,the full plate current starts to flow instantly. This plate currentdevelops an IR drop across the resistance 3 instantly. This IR drop lsalso impressed across the resistances 4 and 'I and condenser 5, that isconnected in series with them.

This results in the point B going instantly to a.

maximum negative value with respect to the ground 6. Since the controlgrid has ths value of IR drop impressed upon it, the gain in amplicationinstantly drops to a predetermined value.

Then, as the current continues to flow through the resistances 4 and 1,the condenser 5 will become fully charged and the gain in amplification,controlled by the IR drop in the res'stance, will increase exponentiallywith time.

In Figure' 9 there is shown a further modification of this invention, inthat the fundamental circuit is used to control the grid bias voltage onvacuum tubes in a plurality of amplifiers. In this illustration. of thecircuit diagram the potential at point B through the resistances I2 isappl'ed to the grids of one or more vacuum tubes in aplurality ofamplifiers. The resistances I2, in addition to serving as means wherebya' .connection to a plurality of amplifiers can be made, are madevariable so that the maximum gain in amplification for each amplifiercan be controlled independently of the others.

The battery'IS in this circuit is used to adjust the initial and/ormaximum amplification of all the amplifiers controlled. The potential ofthe battery 8, by means of the variable resistances I2, is used tocontrol the initial and/or maximum gain in amplification of theindividual amplifiers. With such an arrangement it is possible to recordon one seisrnogram several traces having equal amplitudes throughouttheir length. Although the illustrative circuits by which the control ofthe amplifier is accomplished, are described in connection with thesuppressor grid of a conventional vacuum tube, it is obvious that equalor even better c'cntrol ,can be had by ,applying this principle to theordinary control grid of three element tubes.

, ducted through the conductor I4l to` the trans-v Referring to Figure9, the operation of this system is as follows: A signal suchas thatdelivered by the impulse recorded on the last trace of the seismogramdue to directtraveling waves is conformer T and is impressed upon thegrid of a gas 'triode tube I3, which has been adjusted to some negativevalue.

closed when this impulse is impressed upon the grid of the gas trode,the arc will be struck, due i to the fact that the grid has been madesufficiently positive to cause it to lose control. A flow of platecurrent results through the circuit, which will in turn flow throughvtheresistances 4 and 'l to charge the condenser 5. At the time the platecurrent begins to now inthe plate' circuit of this Assuming that theswitch I0 is some predetermined value.

tial at the point B will go from some maximum negative value to groundpotential at the time the condenser becomes fully charged. This variable potential produces a change in the grid bias on the vacuum tube 9.There will result a corresponding change in the gain in amplification.In Figure 4, the instantaneous values of amplication, from the instantat which the time break occurs until the entire seismogram is recorded,are illustrated. Utilizing a system having a characteristic curve ofthis nature will produce a seismogram such as that illustrated inFigures 2a and 2b. A study of this seismogram will indicate that no morethan one record is required to record all the data necessary on aprofile that is shot in one direction.

In Figure 9 it will be noted that by connecting the conductors l to thetime break and telephone circuit that the operation of this circuit canbe initiated by the time break. With the circuit in an electricseisrnograph connected in this manner it will function as an ordinaryexpander at the time the time break impulse is received at thetransformer I. The grid of the gas tricde I3 will immediately losecontrol and plate current will begin to flow. `As described above, thisplate current will develop a large IR drop across the resistances d andto place a relatively large negative bias upon the vacuum tube 9 in theconventional amplifier shown. This results in a suppression in the gainin amplication instantly to As this occurs the gain in amplification isthen allowed to build up exponentially with time as the condenser 5becomes fully charged.

Although specic arrangements of elements are herewith described, it isobvious to those skilled in t the art that other arrangements ofelements in an electric circuitwould produce the same result. t Althoughthe circuit of Figure 9 shows the control acting on a grid of one tube,it is equally adaptable to the control of the grids of more than onetube of a seismograph amplier.

I claim:

l. Apparatus for making records of seismic waves generated at ashot-point and received at a station remote therefrom,'comprising aseismometer at the said station adapted to convert said waves intoelectro-motive-forces o-f voltages indicative of the amplitude and insympathy with vsaid waves,- an amplifier for magnifying said voltages,a-galvanometer-recorder for recording the same in respect to timeintervals, and means responsive only oncey during the receipt of saidwaves to an amplifier loutput above a predetermined minimum valueftoreduce the gain of said amplifier, and means to gradually return saidgain to normal over a period of the order of the duration of the waves.

2. Apparatus for making records of seismic vv'aves generated at ashot-point and received at a station remote therefrom, comprising aseismometersat the said station adapted to convert said waves intoelectro-motive-forces of voltages indicative of the amplitude and insympathy with the same in respect to time intervals, means responsiveonly once during vthe receipt of said Waves toan amplifier output abovea predetermined minimum value, to reduce the gain of said amplier, meansto gradually return said gain to normal over a period of the order ofthe duration of the waves, and means to adjust the rate of return oisaid gain to normal.

3. Apparatus for making records of vseismic waves generated at ashot-point and received at a surface station remote therefrom,comprising a seismometer at the said station adapted to convert saidwaves into electro-motive-forces of voltages indicative of the amplitudeand in sympathy with said waves, an amplier for magnifying saidvoltages, a galvanometer-recorder for recording the same in respect totime intervals, means responsive only once during the receipt of saidwaves, to an amplifier output which is greater than a predeterminedvalue, to reduce the gain of said amplifier, means to gradually returnsaid gain to normal over a period of the order of the duration of thewaves, and means to nonlinearly change the rate of return of said gainto normal.

4. A recording system for seismic waves occurring in a train extendingover a period of several seconds in which thejrst part of the train hasa general amplitude level several times Ygreater than that of theremainder and in which the general level of the remainder graduallydecreases, comprising in combination, a detector for converting saidwaves into voltages indicative of the amplitude and in sympathy withsaid waves, an amplier for said voltages associated with said detector,a recorder connected to the output of said amplifier, said amplierincluding a tube having a control element, a control tube having acontrol element associated with said output and a cathode, a source ofnegative bias voltage for said second mentioned element having itspositive side at 'cathode potential and being adjusted in value toprevent currentA `ilow in the anode circuit of the control tube untilsaid output exceeds a predetermined value, a resistor -in the said anodecircuit the drop across which increases the amount of negative bias onsaid amplifier tube control element when current flows through theresistor, and means in said anode circuit to effect an initial highcurrent flow through the resistor followed by a gradual diminution inthe current.

, 5. In methods of seismograph prospecting wherein a series'o seismicvibrations of amplitude initially high and decreasing with time aredetected, converted into electrical signals and the signals areamplified electrically and recorded,

. energy the controller acts to lower the gain and thereafter graduallyand progressively restores the gain: the .improvement which comprisescutting off the effect of the signal energy on the -gain controllerimmediately after receipt of the initial high amplitude signals andkeeping such effect cut on throughout receipt of the remainder of theseriesof vibrations, whereby the gain increases smoothly without fallingoff to lower values, subsequent to receipt of the initial high amplitudesignals, uninuenced byl subsequent fluctuations in amplitude of signalenergy.

.6. An apparatus-for seismic prospecting comprising a detector of;seismic waves adapted to convert said waves into electrical signals; a

ther-imams tube ampnfief for ampnfying said electrical signals, saidtube amplifier having a rcontrol grid; a recorder for recording saidelectrical signals in correlation with time; a control circuit includinga Thyratron tube, a resistance and a condenser in series in the platecircuitl of said Thyratron tube; means for impressing a negative bias onthe control grid of said amplifier upon the flow of current in saidplate circuit whereby the gain of said amplifierv Thyratron control gridto prevent the flow of current in the plate circuit of the Thyratrontube; and means for reducing the bias on said Thyratron control grid topermit the ow of current in said Thyratron plate circuit.

7. An apparatus for seismic prospecting comprising a detector of seismicwaves adapted to convert said waves into electrical signals; athermionic tube amplier for amplifyingr said electrical signals, said-tube amplifier having a control grid; a recorder for recording saidelectrical signals in correlation with time; a control circuit includinga Thyratron tube, a resistance and a condenser in series in the platecircuit of said Thyratron tube; means for impressing a negative bias onthe control grid of said amplier upon ,the ow of current in said platecircuit whereby the gain of said ampliiier is reducedA substantiallyinstantaneously upon the flow of current in said plate circuit and isgradually restored upon the completion of the charging of saidcondenser; a control grid for said Thyr'atron tube; means for normallybiasing said 'I'hyratron control grid to prevent the now of current inthe plate circuit of the Thyratron tube; and means responsive to theinitial output of signals from said amplier for reducing the bias onsaid Thyratron control grid to permit the i'low of current in saidThyratron plate circuit.

i3. Apparatus for seismic prospecting comprising a detector of seismicwaves adapted to convert said waves into electrical signals; athermionic tube amplifier to amplify the electrical signals, said tubeampliier having a suppressor grid; a recorder to make a record of saidelectrical waves in correlation with time; a control circuit includingin series a condenser, a resistance, a source of current and a Thyratrontube; a connection between said suppressor grid and said circuit toimpose a negative bias on said grid when the circuit is initially closedby the striking of an arc across said 'I'hyratron and thereby'substantially instantaneously reduce the gain of said thermionic tubeamplifier, said ,neg`

ative bias gradually decreasing with the redu'c tion in the current insaid. circuit as said condenser becomes charged and thereby graduallyrestoring the gain of said thermionic tube amplifier; and meansresponsive to the seismic operations for causing an arc to strike acrossthe Thyratron tube.

9. Apparatus for seismic prospecting comprising a detector of seismicwaves adapted to convert said waves into electrical signals; athermionic tube amplifier to amplify the electrical signals, said tubeampler having a, suppressor grid; a recorder to make a record of saidelectrical waves in correlation with time; a control circuit includingin series a condenser, a re,-A

sistance, a source of current and a Thyratron tube; a connection betweensaid suppressor grid and said circuit to impose a negative bias on saidgrid when the circuit is initially closed by the striking of an arcacross said Thyratron and thereby substantially 4instantaneously reducethe gain of said thermionic tube amplifier, said.

10. An apparatus for seismic prospecting com` prising a detector ofseismic vwaves adapted to convert said waves into electrical signals, athermionic tube amplifier for amplifying said electrical signals, saidamplifier having a control grid, a recorder for recording saidelectrical signals in correlation with time, a control circuit for saidampliiiei` including a resistance and a condenser in series, means forapplying a potential difference across said series-connected resistanceand condenser thereby to produce current flow through said resistorwhich is initially of high magnitude and as said condenser is charged isgradually reduced, and means for applying to said control grid anegative bias derived from the potential' difference produced acrosssaid resistance by the current ow therein, whereby the gain of saidamplifier is reduced substantially instantaneously upon the flow ofcurrent through said control circuit and is gradually restored upon thecompletion of the charging of said condenser.

l1. An apparatus for seismic prospecting com prising a detector ofseismic Waves adapted to convert said waves into electrical Signals, athermionic tube amplifier for amplifying said electrical signals, saidamplifier having at least one gain-controlling grid, a recorder forrecording said electrical signals in correlation with l time, a controlcircuit for said ampliiier including a resistance and a condenser inseries, means operable during receipt of said waves for applying apotential difference across said seriesconnected resistance andcondenser thereby to produce current iiow through said resistor which isinitially of high magnitude and as said conl denser is charged isgradually reduced, and means for applying to said control grid anegative bias derived from the potential diierence produced across saidresistance, whereby the gain of said amplifier is reduced substantiallyinstantaneously upon the ilow of current through said control circuit,and is gradually restored to its original value upon the completion ofthe charging of Vsaid condenser.

12. In an apparatus for seismic prospecting having a detector of seismicwaves adapted to convert said waves into electrical signals, athermionic tube amplifier for amplifying said electric signals, saidampliiier having at leastI one gain-controlling grid and a recorder forIing on said gain-controlling grid of said amplifiera negative bias whosemagnitude varies with that of said current flow, thereby quickly toreduce by a substantial amount the gain of said amplifier and thereaftergradually to increase the amplification to its initial value upon thecompletion of the charging of said condenser.

13. In an apparatus for seismic prospecting` having a detector ofseismic waves adapted to convert said waves into electrical signals, athermionic tube amplifier for amplifying said electric signals, saidamplifier having at least one gain-controlling grid and a recorder forrecording said electrical signals in correlation with time, thecombination of means for controlling the amplification of said amplifiercomprising a control circuit including a resistance and a condenser inseries, means operable at a predeterminedtime during the detection ofsaid waves for applying a substantial potential difference to saidcontrol circuit to produce a current flow therein which is initially ofhigh magnitude and which decreases with the, charging of said condenser,means for producing on said gain-controlling grid of said amplier a,negative bias whose magnitude varies with that of said-current now,thereby quickly toreduce by a.

fication to its initial valueupon the completion of the charging o-fsaid condenser.

14. In an apparatus for seismic prospecting' having a detector ofseismic waves adapted to convert said lwaves into electricalv signals, athermionic tube amplifier for amplifying said electric signals, saidamplifier having at least one gain-controlling grid and a recorder forrecording saidelectrical signals in correlation with time, thecombination of means'for controlling the amplification of said amplifier'comprising a control circuit including a resistor and a condenserA inseries, means :for applying a substantial potential diierence to saidcontrol circuit.

to produce a current ow therein which is initially of high magnitude'andwhich decreases with the charging of said condenser at a predeterminedrate fixed by the capacity ,of said condenser and the resistance of saidresistor, means for producing .in response to said current flowafnegative bias'on said gain-controlling grid of V`said ampliiiertherebyquickly to reduce by a predeterminedamount the gain of saidamplifier, and thereafter gradually to increase the amplification to itsinitial value upon the completion of the charging of said condenser, andmeans for adjusting the extent of said reduction in amplificationwithout changing the resistance 0f M. sHooK.

